It is known that, in a CDMA mobile communications system, a signal transmitted from a base station will arrive at UE via different paths after different time delay because there are reflections of electro-magnetic waves. With matched filtering, UE will be able to obtain the multi-path profile. Likewise, a signal transmitted from UE will reach the base station via different paths. After matched filtering, the base station will also be able to obtain the appropriate multi-path profile.
The locations of several larger peaks in the multi-path profile are usually corresponding to the different propagation paths (simplified as path) of the signal, where the path with the shortest delay in the multi-path profile is referred to as the first path. It is seen from the above definition that the first path is corresponding to the shortest propagation path of signal between the base station and UE. Thus the purpose of first-path detection is to determine the location of the first path in the multi-path profile after the profile thereof is obtained.
First-path detection is the prerequisite of implementing UE positioning based on a cellulous mobile communications system, and the precision of UE positioning directly depends on the precision of first-path detection. For example, the WCDMA-based OTDOA-IPDL (Observed Time Difference Of Arrival-Idle Period Down Link) positioning method requires the measurement of some radio signals by UE or the base station, and the measurement includes SFN-SFN (System Frame Number) OTDOA Type 2 measurement at the UE side, UE Rx-Tx time difference Type 2 measurement, and the measurement of Round Trip Time (RTT) at the base-station side. Obtaining the values of the above measurements all depend on the first-path detection of the corresponding radio signals by UE or the base station. That is why the precision of first-path detection determines the precision of the values of the above measurement, and accordingly the positioning precision of this kind of UE positioning methods based on a wireless cellulous network.
In the past, the ordinary method of first-path detection is implemented in two steps. The first step: obtain the noise threshold of the multi-path profile; the second step: search the first maximum point exceeding the noise threshold in the multi-path profile, and the location of this maximum point will be determined as the first path.
The foregoing ordinary method of first-path detection takes no consideration of the impact of the following two factors on first-path detection: side-lobe and first-path ambiguity, leading to a larger error in the first-path detection in a relatively complicated radio propagation environment.
Typically, in a CDMA mobile communications system, transmitting and receiving radio signals will be implemented by low-pass shaping filtering, for instance, the RRC (Root Rising Cosine) filtering in a WCDMA system. The shape-filtered signal has un-ignorable side-lobes, which are other maximums in addition to each path in the shape-filtered signal and will appear on the left and right sides of each path after matched filtering at the receiving end of the radio signal. If a side-lobe at the left side of the first path exceeds the noise threshold, as shown in FIG. 1, the side-lobe at the left side of the first path will be falsely detected as the first path by the ordinary method, leading to a significant first path detection error.
In addition, there is the multi-path effect in the propagation of radio signals. When the amplitude, phase, and path spacing between different paths meet certain conditions, typically when there is a relatively small path spacing between the first path and the second path, for instance, the spacing of two paths less than two chips (corresponding to 160 meters), or when the second-path's power is equal to or stronger than the first-path's power, the second path will blur the maximum that the first path should have had, i.e. making it no longer a maximum. This is what is called first-path ambiguity. As shown in FIG. 2, the first path is ambiguous by the following second path, leading to the disappearance of the maximum at the first-path location. As a result, by the ordinary method of first-path detection, the following maximum (second path) will be falsely regarded as the first-path location, leading to a significant first path detection error.
Apart from that, in a digital CDMA mobile communications system, the received signal will be sampled in a finite rate. With the ordinary method of first-path detection, the first-path location is detected at the location of the maximum in the multi-path profile with discrete sample values, therefore, the first path thus detected can only be located at the discretely sampled maximum point. The resolution of the first-path depends on the spacing of the sampling points: the higher the sampling rate, the smaller the spacing between the two sampling points, and the higher the resolution of the first-path, making the detected points closer to the real first-path location. With the ordinary method of first-path detection, however, only finite rate first-path resolution can be achieved.